Polyacrylamide is used in aqueous fluids for displacing oil through crude oil containing reservoirs. Sufficient polyacrylamide is included in the aqueous fluid to reduce the mobility of the fluid in the reservoir and thereby improve the sweep or displacement of fluids through the reservoir. The polymer-containing aqueous fluid can be referred to as a polymer bank or mobility buffer bank. The polymer bank is used for directly displacing crude oil through a reservoir or displacing surfactant fluid such as a micellar fluid through a reservoir.
Water that is commercially available for the formulation of the polymer bank can be a hard brine with a high salinity. Such brines can contain greater than about 500 milligrams/liter (mg/l) of divalent ions and have greater than about 5,000 mg/l of total dissolved solids. Some brines from which polymer banks are formulated can have greater than about 1,000 mg/l of divalent ions and greater than about 10,000 mg/l of total dissolved solids.
Salinity of a brine, as indicated by the total dissolved solids, and the hardness of the brine, as indicated by the divalent ion concentration, are significant factors in the efficiency with which a polymer bank formulated with the brine can displace fluids through a reservoir. An increase in the salinity and hardness tends to decrease the resistance to movement of the polymer bank through a reservoir. It has been observed that divalent ions which contribute to hardness and salinity have a greater effect on decreasing resistance to movement than do monovalent ions which only contribute to salinity. Therefore, changes in divalent ion concentrations have greater effects on resistance to movement than equivalent changes in monovalent ion concentrations.
The resistance to movement of a polymer bank in a reservoir can be described in terms of a resistance factor which reflects the decrease in the mobility of a polymer bank through a reservoir in comparison with the mobility of the aqueous fluid with which the polymer bank is formulated. The permeability of the reservoir to the aqueous fluid used in formulating the polymer bank and to the fluids constituting the polymer bank, as well as the viscosities of the aqueous fluid used in formulating the polymer bank and the fluid constituting the polymer bank, are included in the resistance factor calculations. Measurements made with a screen viscometer have been found to correlate closely to such resistance factor calculations. This screen viscometer has a fluid volume of about 30 ml and consists of a pack of five 0.25-inch (0.635-centimeter) diameter stainless steel screens. The screens are 100 mesh USTM standard screen size. The flow time of fresh water through the viscometer is about 8 to 10 seconds. Prior to passing the fluid through the viscometer, it is filtered through a 200 mesh screen.